The invention relates to a process for selective removal and destruction of hydrogen cyanide (HCN) from a mixture with nitrous-containing gases.
On oxidation of alkylated aromatics with nitric acid, in addition to nitrous gases HCN also forms in the waste gas. The nitrous gases NO and NO.sub.2 can be used for production of nitric acid, but HCN must first be removed from the waste gas. A number of processes are known for removal of HCN from waste gases of different compositions. A process for washing NH.sub.3, H.sub.2 S and HCN out of gases by means of iron(II) salt solutions containing sulphuric acid is thus known from DE-PS No. 1,121,268. This gas washing is carried out in three stages at increasing pH values, corresponding partly insoluble salts which must subequently be disposed of being formed in each case with the gas constituents. It is furthermore known from DE-OS NO. 3,327,592 that HCN can be removed from gas mixtures via an active charcoal fixed bed catalyst charged with sulphur, thiocyanates being formed. The purification of the catalyst contaminated with thiocyanates and disposal of these thiocyanates is not described. The possibility of removing HCN from acid waste gases, which also contain H.sub.2 S, SO.sub.2 and CO.sub.2, by alkaline absorption is furthermore known from U.S. Pat. No. 2,747,962. Salts which must in turn be disposed of are formed here.
The object of the present invention was thus to discover a process in which HCN can be removed from nitrous-containing waste gases so that no products which subsequently had to be disposed of are obtained. This was to be achieved by a simple process in which HCN is removed from the waste gas by washing this with nitric acid in counter-current, so that the NO-- and NO--.sub.2 -containing waste gas which remains can be processed quantitatively to give nitric acid. The HCN absorbed by the nitric acid is then hydrolyzed and oxidized during regeneration of the washing acid at elevated temperature.